The present application is directed to carbon capture and regeneration, and is more particularly directed to solvent based capture and regeneration of CO2 generated from fossil fuel fed electricity generating facilities, coal gasification plants or other sources.
While solvent based capture techniques hold promise they are not without drawbacks. Capture and regeneration energy efficiencies are still low in relation to what is desired for power plant flue gas extraction, pre-combustion gas extraction from coal gasification plants, and extraction of CO2 from other sources. Solvent cost and degradation of system components have also been identified as concerns. Still further, the potential emission of the solvents and solvent degradation byproducts need to be addressed.
Inorganic aqueous methods of CO2 capture have certain advantages and are used where process conditions allow, and the value of the product is sufficiently high to justify the increased cost. One particular type of inorganic aqueous method is known as the Benfield high temperature aqueous KCO3 capture system. This process is appropriate where organic solvent use is not desired and cost is not problematic. The Benfield system is one of the solvent capture technologies commonly used in Natural Gas and Petroleum Refining industries for CO2 removal.
While the aqueous capture of CO2 taught by the Benfield system is fast and efficient, the energy consumed during regeneration is large and pressurization of the input gas stream is required making it unattractive for flue gas capture. Therefore, alternate, more efficient systems and methods of aqueous solvent regeneration are needed.
One particular innovation in the area of aqueous capture of CO2 is the employment of Bipolar Membrane Electrodialysis (BPMED), which has been used in acid/base recovery and/or other conditioning of waste streams. BPMED takes advantage of the natural dissociation of water into hydroxyl and hydronium ions in the presence of an external field to generate separate acidic and basic aqueous streams. BPMED has been investigated for use in CO2 recovery from flue gas by Nagasawa, et al., in the publication, Nagasawa, H, Yamasaki, A., Yanagisawa, Y, NETL-Sixth Annual Conference on Carbon Capture & Sequestration (2007), which is hereby incorporated by reference in its entirety.
Turning to FIG. 1 illustrated is a system 100 for CO2 and alkaline solution recovery from alkaline carbonate solution employing an electrodialysis system and method such as proposed by Nagasawa, et al.
In implementing an electrodialysis CO2 recovery process in accordance with the system of FIG. 1, the following reactions occur in an aqueous solution of an alkaline carbonate (M2CO3) in contact with CO2 in the gas phase:CO2+H2OH2CO3   (1)H2CO3H++HCO3−  (2)HCO3−H++CO32−  (3)
The equilibrium could be shifted to the left-hand side to increase the CO2 partial pressure either by increasing the temperature or adding protons (i.e., decreasing pH) into the system. The former mechanism corresponds to a thermal recovery process of CO2 from carbonate solutions. The process to be described corresponds to the latter mechanism. Particularly, the protons can be supplied by the dissociation of water molecules, as in:H2OH++OH−  (4)
Prior to being able to recover CO2 protons need to be separated from hydroxyl ions and then be supplied to a feed solution. Next, to keep electro-neutrality of the solution, alkaline metal ions in the feed solution should be removed. The removed hydroxyl ions and alkaline metal ions will form the alkaline solution that can be reused for CO2 absorption.
To achieve the above process, and as shown in system 100 of FIG. 1, a base cell 102 and a feed cell 104 are sandwiched by two bipolar membranes 106,108 and one cation exchange membrane 110. The other sides of the bipolar membranes 106,108 being placed in contact with a respective cathode electrode cell 112 and anode electrode cell 114, where electrodes 116,118 are inserted in corresponding electrolyte solution 120,122. A carbonated alkaline solution 124 is fed through feed cell 104, and an electrolyte solution 126, with the same cation as alkaline solution 124, is fed through base cell 102. The release CO2 is shown at 128.
Cation exchange membranes are a type of ion exchange membrane that can exclusively transport cations. The bipolar membrane has a laminated structure of two layers, a cation exchange layer and an anion exchange layer (not shown individually).
When an electric potential difference larger than the electrodialytic splitting voltage of water is applied via a power source (not shown) to electrodes 116,118, the bipolar membrane 108 splits water molecules into pairs of proton (H+) and hydroxyl ions (OH—). The produced protons are transported into feed cell 104 according to the potential difference. At the same time, the alkaline metal ions (M+) in feed cell 104 are transported into base cell 102 through cation exchange membrane 110. As a result, the pH of the feed solution is decreased, and CO2 gas is generated from the solution according following mechanism:CO32−+H+→HCO3−  (3′)HCO3−+H+→H2CO3   (2′)H2CO3→CO2↑+H2O   (1′)
On the other hand, bipolar membrane 106 in contact with base cell 102 supplies hydroxyl ions (OH—) to the base cell, where the alkaline solution is regenerated with the alkaline metal ions (M+) transported from feed cell 104. The regenerated alkaline solution can be reused for CO2 capture from exhaust gas.
Electrodialysis is a potentially energy-saving process because it can be operated near thermodynamic minimum energy consumed. However, the process efficiency will be significantly reduced by the electrolysis processes in the electrode cells. For example, some part of the electric power will be consumed for producing oxygen and hydrogen gases at the dialysis cell electrode terminals.
However, the amortized energy consumption of the electrolysis will be reduced and the efficiency of the process improved by increasing the number of the pairs of feed cells and the base cells between two electrodes. More particularly, the power consumed by electrolysis (H2 and O2 formation at the electrode terminals) is constant at constant current irrespective of the number of pairs of feed cells and base cells between the two electrodes. With an increase in the pair of the base and feed cells, the energy consumed by electrolysis per each cell will be decreased.
However, there are drawbacks to the system described by Nagasawa et al. Among the obstacles to employing BPMED technology in the recovery of CO2 has been the potential for the physical degradation of the system due to operational concerns, such as evolving of the CO2 while in the cells, as well as capital and operating cost, such as energy costs due to uncontrolled system pH. These drawbacks are not fundamental and it is therefore considered useful to present improvements to existing concepts to overcome these limitations to carbonate/bicarbonate extraction and CO2 concentration.